For the past several years, our group has been investigating the detailed structure and function of a class of Cys4Fe2S2 ferredoxins which act as reductants and effectors of cytochrome P450 monooxygenases, enzymes which reduce one oxygen atomof 02 to water while inserting the second oxygen into an unactivated C-H bond in a substrate. Proteins of the P450 superfamily are widely distributed in nature from aerobic bacteria to mammals. In many of these systems, the cytochrome P450 accepts electrons from ferredoxins, which in turn are reduced by NADH or NAD(P)H-dependent flavoproteins. The interactions between redox proteins in these systems tend to be quite specific, with low cross-reactivity between homologous systems. For example, there is a high degree of structural and sequence homology between putidaredoxin (Pdx) and adrenodoxin (Adx), two ferredoxins which donate electrons to bacterial and mammalian cytochromes P450, respectively. However, Adx is not competent to transfer the second electron required for turnover to cytochrome P450, the physiological redox partner of Pdx. Even the closely related terpredoxin (Tdx) which is a bacterial protein with a high degree of sequence, functional and structural homology to Pdx. shows only 10% cross-reactivity with the P450,,. system. To further explore these structure-function relationships, we are determining the structure of terpredoxin (Tdx). We have obtained 750 MHz high resolution 'H,'H-NOESY, 1H, "N-TOCSY-HSQC and 'H, "N-NOESY-HSQC data, which we have analyzed to yield nearly complete assignment of 1H and "N resonances for the diamagnetic region of Tdx.